I just recalibrated my camera in indoor lighting and found that for indoor lighting it is off by 2 stops - ISO400 film needs to be set at ISO1600. So in daylight it is -3 stops form actual reading and in tungsten it is -2 stops.

Ditto on battery thoughts. Modern 'replacements' are not mercury cells and have slightly lower voltage.

The FA uses standard SR44 batteries.

Its possible the ASA base plate under the rewind knob needs adjustment.

You really should use a light meter for comparisons.

One trick I use on older electronic cameras is to turn the ev adjustment back and forth a few times and the ASA dial a few times along with the aperture ring. Contacts although gold can get a film of foreign material on them causing eratics readings.

Another thing you might consider is the lens type you have mounted. I've never had an FA but it may not play nice with G type lenses and/or pre-AI lenses. I honestly don't know how issues related to non-compatible lenses would manifest themselves on the FA. It may not matter. Have you checked for damage to the AI follower.